Method, device, and system for estimating nugget diameter in resistance spot welding

Disclosed is a nugget diameter estimation method provided with: an acquisition step for acquiring, for each welding point, time series data including data before the start of energization, with respect to two or more welding parameters including swell amounts calculated on the basis of the displacement of an upper electrode and a lower electrode; a difference calculation step for calculating: (i) a first difference, which is the difference between the swell amount at a first point before the start of energization and the swell amount at an Nth point before the start of energization, where N is an integer of 2-M; and (ii) a second difference which is the difference between the welding parameter at a first point after the energization start time and the welding parameter at an Nth point after the energization start time; and an estimation step for estimating the nugget diameter of the Pth point using the welding parameters, where P is an integer of 1-M inclusive. 本发明公开了一种熔核直径的推定方法，具备：取得工序，关于包含基于上部电极和下部电极的位移算出的膨